Document that web pixel events are not relayed to the host app

[tiagocandido]

What changes are you making?

Checkout Kit dropped the Checkout Sheet Kit web pixel relay APIs (checkoutDidEmitWebPixelEvent, onWebPixelEvent, the React Native pixel listener) in #81, but we never documented it. Apps migrating from Checkout Sheet Kit have nothing telling them to remove those handlers or where pixel behavior went.

This adds a short migration note to the root README (Versioning section) and a one-liner to the Swift, Android, and React Native lifecycle sections: pixel events aren't relayed to the app, and merchant web pixels run inside checkout automatically.

How to test

Docs only — read the updated README sections.

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Before you merge

Important

• I've added tests to support my implementation (n/a — docs only)
• I have read and agree with the Contribution Guidelines
• I have read and agree with the Code of Conduct
• I've updated the relevant platform README (platforms/swift/README.md and/or platforms/android/README.md)